ISO 20975-2:2018

INTERNATIONAL ISO
STANDARD 20975-2
First edition
2018-09
Fibre-reinforced plastic
compositess — Determination
of laminate through-thickness
properties —
Part 2:
Determination of the elastic
modulus, the strength and the
Weibull size effects by flexural test of
unidirectional laminate, for carbon-
fibre based systems
Composites plastiques renforcés de fibres — Détermination des
propriétés de l'épaisseur traversante d'un composite stratifié —
Partie 2: Détermination du module d'élasticité, de la résistance et
des effets de la taille de Weibull par essai en flexion sur un composite
stratifié unidirectionnel, pour systèmes à base de fibres de carbone
Reference number
©
ISO 2018
© ISO 2018
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ii © ISO 2018 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
5 Conditioning . 3
5.1 Conditioning of test specimen . 3
5.2 Temperature and humidity in testing . 3
6 Apparatus . 3
7 Test specimens. 4
7.1 Shape and dimensions . 4
7.2 Preparation of specimens . 4
7.2.1 General. 4
7.2.2 Test plate . 4
7.3 Specimen inspection . 5
7.4 Number of specimens . 5
8 Procedure. 6
9 Calculations. 6
9.1 Through-thickness tensile stress . 6
9.2 Through-thickness tensile strength . 7
9.3 Weibull modulus by flexural test . 7
9.4 Effective volume of flexural specimen . 7
9.5 Through-thickness tensile strain . 8
9.6 Through-thickness tensile strain at failure . 8
9.7 Through-thickness tensile modulus . 8
9.8 Significant figures of stress and elastic modulus . 8
9.9 Expression of results . 9
10 Precision . 9
11 Test report . 9
Annex A (informative) Precision data obtained from an interlaboratory test .11
Bibliography .13
Foreword
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This document was prepared by Technical Committee ISO/TC 61, Plastics, Subcommittee SC 13,
Composites and reinforcement fibres.
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iv © ISO 2018 – All rights reserved

Introduction
With the increasing number of applications of composite materials, the necessity of a test method
[1]
for evaluating through-thickness mechanical properties has been recognized. ASTM D6415 and
[2]
ASTM D7291 were proposed for evaluating out-of-plane properties. However, ASTM D6415 has
no method for evaluating through-thickness tensile modulus because this test method is based on
evaluating the curved beam strength of a fibre-reinforced polymer matrix composite laminate panel.
[2]
Further, the following problems have been reported regarding ASTM D7291 ; bonding between the
specimen and the loading tab is required, the stress distribution in the specimen is not uniform, and a
[3],[4]
local stress concentration is generated . This means that the fracture load depends on the maximum
stress induced at the point of the stress concentration. Therefore, through-thickness strength evaluated
by ASTM D7291 cannot be simply compared with other specimens. In addition, it was reported that the
apparent out-of-plane modulus evaluated under ASTM D7291 varies with strain gage position, strain
[5]
gage length, and specimen thickness . This document provides a test method to evaluate the through-
thickness mechanical properties of composite materials.
INTERNATIONAL STANDARD ISO 20975-2:2018(E)
Fibre-reinforced plastic compositess — Determination of
laminate through-thickness properties —
Part 2:
Determination of the elastic modulus, the strength and
the Weibull size effects by flexural test of unidirectional
laminate, for carbon-fibre based systems
1 Scope
This document specifies a flexural test method for determining the through-thickness (out-of-plane)
tensile properties of laminated carbon fibre-reinforced plastic (CFRP) composites, including strength,
fracture strain, and modulus. This document is applicable to unidirectional CFRP (UD-CFRP) laminates.
In addition, the calculation of effective volume is also described due to size effects of the through-
thickness tensile strength.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 291, Plastics — Standard atmospheres for conditioning and testing
ISO 1268-1, Fibre-reinforced plastics — Methods of producing test plates — Part 1: General conditions
ISO 1268-4, Fibre-reinforced plastics — Methods of producing test plates — Part 4: Moulding of prepregs
ISO 2818, Plastics — Preparation of test specimens by machining
ISO 5893, Rubber and plastics test equipment — Tensile, flexural and compression types (constant rate of
traverse) — Specification
ISO 20501, Fine ceramics (advanced ceramics, advanced technical ceramics) — Weibull statistics for
strength data
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
3.1
span length of flexural test
L
length of span between supports
3.2
flexural load
P
load on specimen from loading nose at any given time
3.3
through-thickness tensile stress
σ
nominal stress in the outer surface of the flexural test specimen at mid-span
Note 1 to entry: It is calculated according to the relationship given by Formula (2).
3.4
deflection
D
distance through which the top or bottom surface of the test specimen at mid-span has deflected during
flexure from its original position
3.5
through-thickness strain
ε
nominal fractional change in length of an element in the outer surface of the test specimen at mid-span
3.6
through-thickness tensile strength
σ
f
maximum through-thickness tensile stress by flexural test or through-thickness stress at failure
3.7
through-thickness modulus
E
slope calculated from linear region of through thickness stress–strain curve
3.8
Weibull modulus of through-thickness strength data
m
empirical parameter that decides the probability density function of the Weibull distribution
3.9
effective volume of flexural specimen
V
eff
size of an equivalent uniaxial tensile specimen that has the same probability of rupture as the test
specimen
Note 1 to entry: The effective volume is parameter of Weibull size effects of strength. It is calculated from the
thickness, the width and the span length of the flexural test, and the Weibull modulus based on beam theory.
4 Principle
A rectangular test specimen, whose longitudinal direction coincides with the through-thickness
direction of the CFRP laminate, is positioned between two supports and deflected by means of a loading
edge acting on the specimen midway between the supports. The test specimen is deflected at a constant
rate until rupture occurs on the outer surface of
...